1. Field of the Invention
This invention relates to a method for fabricating solid electrolytic capacitors wherein a conductive polymer layer is used as the solid electrolyte. The invention also relates to a method for fabricating a solid electrolytic capacitor of the integrated type wherein at least two capacitor elements are integrally combined together in one capacitor unit.
2. Description of the Prior Art
In recent trends toward digitalization of circuits of electric and electronic appliances, there is a strong demand of capacitors for the circuits which exhibit a low impedance in a high frequency range and are compact in size and large in capacitance.
Known high frequency capacitors include, for example, plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. Among them, the film and mica capacitors are so large in size that a difficulty is involved in attaining a large capacitance. The layer-built ceramic capacitors have been developed in order to meet a demand for a large capacitance and a small size of the capacitors, but temperature characteristics become poor, coupled with another disadvantage that such capacitors become very expensive.
Aside from the above capacitors, there are known aluminium dry electrolytic capacitors, and aluminium or tantalum solid electrolytic capacitors.
With aluminium dry electrolytic capacitors, anode and cathode aluminium foils which have been etched, respectively, are convolutely wound through a paper separator, and a liquid electrolyte is impregnated in the separator. This type of capacitor has the serious problem that as time passes, the electrostatic capacitance is decreased along with an increase of the loss due to the leakage and/or evaporation of the liquid electrolyte and the ion conductivity and that the loss in a high frequency range and in a low temperature range is great. To avoid the above problem based on the use of liquid electrolytes, solid electrolytes are employed in the aluminium or tantalum solid electrolytic capacitors.
In the aluminium or tantalum solid electrolytic capacitors, a metallic foil such as an aluminium or tantalum foil which has been anodized to form a dielectric film on the surface thereof is immersed in a manganese nitrate aqueous solution. Then, the manganese nitrate is thermally decomposed at a temperature of approximately 350.degree. C., thereby forming a solid electrolyte layer made of manganese dioxide. Since the electrolyte is solid in nature, the drawbacks, such as deterioration of characteristics involved by the evaporation of the liquid electrolyte at high temperatures or by coagulation of the electrolyte at low temperatures, can be avoided. These solid electrolytic capacitors exhibit better frequency and temperature characteristics than capacitors using liquid electrolytes. In addition, the dielectric film can be made very thin, thus being suitable for fabricating a capacitor with a large capacitance.
In addition, there are known other types of solid electrolytic capacitors which make use of organic semiconductors such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes instead of the manganese dioxide layer as disclosed in Japanese Laid-open Patent Application No. 58-17609 and conductive polymers obtained by electrolytic polymerization of polymerizable monomers such as pyrrole, furan and the like as set forth in Japanese Laid-open Patent Application No. 60-244017.
The solid electrolytic capacitors using the manganese dioxide layer as the solid electrolyte are disadvantageous in that the dielectric film will be damaged during several cycles of thermal decomposition treatments. Additionally, the specific resistance of the manganese dioxide layer is so high that the loss (tan .delta.) in a high frequency range is not satisfactory.
Although the capacitors using the organic semiconductors such as TCNQ complexes have better high frequency characteristics than those capacitors using manganese dioxide but have the problem that the specific resistance is increased at the time of application of the organic semiconductor and that the adhesion of the semiconductor to an anodized metallic foil is not so high. Thus, such capacitors are not satisfactory with respect to characteristic properties.
On the other hand, the capacitors using an electrolytically polymerized material such as of pyrrole, furan or derivatives thereof as the solid electrolyte layer exhibit good frequency and temperature characteristics and a long life, thus being expected as promising.
However, this type of solid electrolytic capacitor has is not satisfactory with respect to leakage current. The reason for this is considered as follows.
For the formation of a conductive polymer layer on the dielectric film, an electrode for initiating the polymerization, which is, for example, a metal electrode having a needle-shaped tip, is contacted with the valve metal from outside. By the contact, the dielectric film is almost invariably damaged. In addition, the contact of the electrode for the initiation of the polymerization from outside makes an undesirably large-sized fabrication apparatus as a whole.
In order to prevent the dielectric film from being damaged, there may be a process wherein a conductive polymer thin film is formed on a valve metal foil, on which a dielectric film has been formed on the surface thereof, by chemical polymerization. Thereafter, part of the conductive polymer thin film is cut off to expose the the valve metal foil. The exposed portion is used as a positive electrode for initiating electrolytic polymerization. However, the exposed portion used as the positive electrode is anodized during electrolytic polymerization and is electrically insulated. Thus, the electric current rarely passes on the way of the formation of the polymer film. This formation of the polymer film becomes very slow. In the worst case, the polymerization reaction will stop.
Further, when it is desired to obtain a capacitor having a very large capacitance, it is usual to build up capacitor elements obtained after completion of the electrolytic polymerization. This eventually leads to a large-size capacitor with a lowering of yield.